Advantage of binocularity in the presence of external visual noise.

PURPOSE: External visual noise plays a major role in real life, for instance, when a driver tries to identify an object through a snow flurry or through a dirty windshield. The goal of the present investigation was to quantify, under such a condition, the advantage of binocular over monocular vision or, rather, the disadvantage of having only one eye. METHODS: Eight observers judged the orientation of a Landolt ring (gap always 10 arcmin), partly obscured by noise particles of different sizes (5, 10 or 20 arcmin). The noise particles were presented at a stereo disparity of 62 arcmin, i.e. beyond Panum's fusional area. We compared the percentage of correct responses and the reaction time between binocular and monocular vision. Control conditions: (1) binocular vision with noise particles located immediately in front of the Landolt ring (stereo disparity +/- 0), and (2) absence of noise particles. RESULTS: With regard to the percentage of correct responses, an advantage of binocular over monocular observation occurred only when the obscuring particles were presented at the stereo disparity of 62 arcmin. The advantage depended on the size of the noise particles. The factor was 1.24 for particles of 5 arcmin, 1.49 for 10 arcmin and 1.59 for 20 arcmin. With regard to the reaction time, there was no difference between binocular and monocular vision. CONCLUSION: Binocular vision provides a considerable advantage over monocular vision when particles partly obstruct the view. This advantage is due to the capability of the visual system to construct a coherent percept of an object of which different parts are imaged in the right and left eye.

Vergence position of rest: spontaneous variability.

BACKGROUND: We determined the vergence position of rest (horizontal component) in two sessions, 1 to 6 weeks apart. Each session contained 8 trials. Twenty observers with normal eyes looked through Risley prisms that allowed a continuous modulation of their strength. We asked the observers to look at a fully fusionable picture, and to adjust the prisms such that viewing appeared most relaxing. The viewing distances were 400 cm and 50 cm. RESULTS: The standard deviation among the 8 trials of each session, averaged over the two sessions, the viewing distances of 400 cm and 50 cm, and the 20 observers, was + or -1.4 prism dioptres (cm/m). From the first session to the second, many observers changed the preferred prism significantly. At a viewing distance of 400 cm, 10 of 20 observers changed their preferred prism by more than 1.0 prism diopter, up to 3.3 prism diopters. At a viewing distance of 50 cm, 11 of 20 observers changed their preferred prism by more than 1.0 prism diopter, up to 5.1 prism diopters. CONCLUSIONS: The prismatic strength estimated as being most relaxing can be very variable. Before considering a prescription, the practitioner should determine whether the preferred prismatic strength remains reasonably stable. To ascertain this consistency, more than one session seems to be necessary.

Microstrabismus: Fixation point scotoma, a perimetric artefact.

The contribution of the strabismic eye to binocular vision has frequently been studied with stimuli presented solely to the strabismic eye, on a binocular background. These studies revealed a central suppression scotoma in the strabismic eye, the so-called fixation point scotoma. Considering that this scotoma might be an artefact due to the unnatural viewing condition, we employed stereoperimetry that allowed examining the contribution of the strabismic eye under natural viewing, and compared the stereoresolution with the Vernier resolution of the strabismic eye. On the retina of the strabismic eye, the stereotarget was imaged at one of seven locations, between 2 degrees nasally and 2 degrees temporally, whereas on the retina of the non-strabismic eye, the stereotarget was imaged always in the centre. The mean stereoresolution of three micro-esotropic observers was 96'', averaged over all seven locations. A reduction of the stereoresolution in relation to the monocular Vernier resolution and to the performance of three non-strabismic observers indicated a slight diffuse suppression, rather than a circumscribed scotoma. We conclude that the strabismic eye contributes more to binocular vision than has been assumed on the basis of tests with targets presented solely to the strabismic eye (on a binocular background).

The prism that aligns fixation disparity does not predict the self-selected prism.

BACKGROUND: Associated phoria is defined as the prism that aligns monocular markers in a fixation disparity (FD) test. We examined whether the 'FD-aligning prism' predicts the 'self-selected prism' (SSP), i.e. the prism that the observer judges to be most comfortable when viewing fully fusionable images. METHODS: For the FD-aligning prism, we employed a test target with a central fusion lock. To determine the SSP, identical images were shown to both eyes. The images were displayed on a cathode-ray tube monitor at a distance of 50 cm, separated for the two eyes by means of liquid crystal shutter goggles. The observer adjusted counter-rotating prisms, recorded with a potentiometer. The procedure was repeated about 1-6 weeks later. RESULTS: Among the 20 non-strabismic subjects over two sessions, there were 16 instances in which the FD-aligning prism indicated a vergence position further in the eso direction than the SSP (< or =15.9 cm m(-1) = prism dioptre). In five instances, the FD-aligning prism indicated a vergence position further in the exo direction (< or =3.6 cm m(-1)). From the first to the second session, changes of > or=1.0 cm m(-1) occurred for the FD-aligning prism in 8/20 observers (< or =11.5 cm m(-1)), and for the SSP in 11/20 observers (< or =5.1 cm m(-1)). DISCUSSION: We hypothesise that the discrepancy between the SSP and the FD-aligning prism may be attributable to the fact that the SSP was determined while the eyes were presented with identical images, whereas the FD-aligning prism was determined while the images for the right and left eyes were different in the area of the Nonius lines. The unusual experience of binocular rivalry around the Nonius lines may have influenced the vergence of the eyes. Considering the spontaneous changes observed within an interval of 1-6 weeks, in both the FD-aligning prism and the SSP, we feel that research on the repeatability of methods for prescribing prisms is indicated.

Do dissociated or associated phoria predict the comfortable prism?

BACKGROUND: Dissociated and associated phoria are measures of latent strabismus under artificial viewing conditions. We examined to what extent dissociated and associated phoria predict the "comfortable prism", i.e. the prism that appears most comfortable under natural viewing conditions. METHODS: For associated phoria, a configuration resembling the Mallett test was employed: both eyes were presented with a fixation cross, surrounded by fusionable objects. Nonius lines served as monocular markers. For dissociated phoria, the left eye was presented with all the Mallett elements, while only a white spot was presented to the right eye. To determine the comfortable prism, all the Mallett elements, including the Nonius lines, were shown to both eyes. In each of the three tests, the observer had to adjust a pair of counterrotating prisms. To avoid any (possibly prejudiced) influence of the experimenter, the prismatic power was recorded with a potentiometer. Twenty non-strabismic subjects with a visual acuity of >or=1.0 in each eye were examined. RESULTS: The range of the intertrial mean was for dissociated phoria from +9.3 eso to -5.9 cm/m exo, for associated phoria from +11.2 eso to -3.3 cm/m exo, and for the comfortable prism from +4.8 eso to -4.1 cm/m exo (cm/m = prism dioptre). In most observers, the phoria parameters differed greatly from the comfortable prism. On average, the phoria values were shifted about 2 cm/m towards the eso direction in relation to the comfortable prism (associated phoria not less than dissociated phoria). CONCLUSIONS: The deviation of both, dissociated and associated phoria, from the comfortable prism suggests that the abnormal viewing conditions under which the phoria parameters are determined induce artefacts. Accordingly, the findings cast doubt on current textbook recommendations to use dissociated or associated phoria as a basis for therapeutic prisms. Rather, patients should be allowed to determine their comfortable prism under natural viewing conditions.